Paper-Based Lithium-Ion Batteries

ABSTRACT

A method for fabricating a paper lithium ion cell including depositing a first lithium-metal oxide composition onto a first electrically conducting microfiber paper substrate to define a cathode, depositing a second, different lithium-metal oxide composition onto a second electrically conducting coated microfiber paper substrate to define an anode, separating the cathode and the anode with a barrier material, infusing the cathode and the anode with electrolytes, and encapsulating the anode, the cathode, and the barrier material in a housing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This utility patent application claims priority to the co-pending U.S.patent application Ser. No. 14/106,759, filed on Dec. 14, 2013, whichclaimed priority to then pending provisional patent application Ser. No.61/737,363, filed on Dec. 14, 2012, now expired.

TECHNICAL FIELD

The present novel technology generally related to electrochemistry, andmore specifically, to batteries.

BACKGROUND

Lithium-Ion batteries are well known in the art. They have been areliable energy source for many electronics and appliances. Typically,lithium ion batteries have good shelf and cycle life, and thus are usedin a wide array of electronics. Their durability and reliability are aprimary reason for the mobile use of smartphones and tablets. However,there are drawbacks that come with conventional lithium ion batteries.Lithium-ion batteries are rigid and prone to fracture, which oftenresults in catastrophic failure, fire and even explosions. Lithium ionbatteries are typically packaged in stiff metal or plastic casings, addbulk weight. Lithium ion batteries that have been developed with a focusto make them less rigid do so at the expense of durability, cycle life,and reliability; these drawbacks manifest themselves almost immediately.Thus, there is a need for a lithium ion battery that is less prone tobrittle fracture and failure, but still remains durable and reliable.The present novel technology addresses this need.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a first schematic illustration of one embodiment of the noveltechnology.

FIGS. 2 A-D is a photomicrograph of the lithium metal oxide materials onthe CNT-fabricated paper substrates, according to the embodiment of FIG.1.

FIG. 3 is a charging curve for Polyimide-polyethylene oxide/lithiumhexafluorophosphate (PEO/LiPF6) solid electrolyte battery test, for theembodiment of FIG. 1.

FIG. 4 is a charging curve of the resultant battery when a few drops ofliquid electrolyte were added to the PEO/LiPF6 solid electrolyte filmsof FIG. 1.

FIG. 5 is graph detailing time vs. voltage of a LTO-PEO/LiPF6/LiCGC(semi-solid)-LCO electrode battery of FIG. 1.

FIGS. 6A-B illustrate a charging curve of a battery when thePEO/LiPF6/LiCGC electrolyte film is dried of FIG. 1.

FIG. 7 is a charging curve of LTO-PEO/LiPF6/LiCGC-LCO battery of FIG. 1.

FIG. 8 is a first photomicrograph image of one embodiment of anelectrode of the embodiment of FIG. 1.

FIG. 9 is a second enlarged photomicrograph of the electrode of FIG. 8.

FIG. 10 is a perspective view of one embodiment of the separator,current collector, and electrolyte, of FIG. 1.

FIG. 11 is a graph of the resistivity wood microfibers measured aftercoating each deposited bi-layer of polymer CNT of FIG. 1.

FIG. 12A is a photomicrograph of uncoated micro-fibers of FIG. 1.

FIG. 12B is a photomicrograph of CNT-coated microfibers of FIG. 1.

FIGS. 13A-B is a photomicrograph of various embodiments of an electrodeof the novel technology of FIG. 1.

FIGS. 14A-B graphically illustrate galvanostatic charging/dischargingcurves for various embodiments of the novel technology of FIG. 1.

FIG. 15A-B graphically illustrate galvanostatic charging/dischargingcurves of various embodiments of the novel technology of FIG. 1.

FIG. 16 graphically illustrates self-discharging behavior of oneembodiment of the novel technology of FIG. 1.

FIGS. 17A-B graphically illustrate galvanostatic charging/dischargingcurves of various embodiments of the novel technology of FIG. 1.

FIGS. 18A-B are photomicrographs of CNT-coated cellulose wood of FIG. 1

FIG. 19 is a partially exploded perspective view of a second embodimentthe novel technology.

FIG. 20 is a graph of the ionic conductivity of PVDF, PEG, PEO, LiTFSIsolid electrolyte mixture of FIG. 1.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thenovel technology and presenting its currently understood best mode ofoperation, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of thenovel technology is thereby intended, with such alterations and furthermodifications in the illustrated technology and such furtherapplications of the principles of the novel technology as illustratedtherein being contemplated as would normally occur to one skilled in theart to which the novel technology relates.

As illustrated in FIGS. 1-20, the present novel technology relates to aflexible, and typically paper-based lithium electrochemical cell system5 with flexible electrode layers 10 and flexible, fibrous electricallyconducting current collecting substrates 15. Typically, the currentcollectors 15 are constructed using particulate conductive nanomaterials20, more typically carbon nanotubes 25, to impregnate or coat wood orcellulosic microfibrous paper 30. The electrode materials are typicallylithium-metal oxide compounds, and, more typically lithium cobalt oxide,lithium titanium oxide, lithium magnesium oxide, lithium manganeseoxide, combinations thereof and the like. The system 5 typicallyconsists of a first lithium metal oxide cathode 35 and a second,different lithium metal oxide anode 40 (i.e. the electrodes 12) with adielectric separator 45 interposed between the two. Typically, theelectrode materials 15 and the nanomaterial-coated wood microfiberpaper-based current collectors 15 experience significantly fewer defectswhen subjected to repeated bending and/or flexure as compared to theconventional electrode materials deposited on conventional metal currentcollectors. Thus, the disclosed paper-based electrodes 55 are wellsuited for flexible battery applications.

Typically, carbon nanotubes (CNT) 25 are used as the conductivenanomaterials 20. Cellulosic or wood microfibers 65 are at leastpartially coated with CNT (such as through layer-by-layer (LbL)nanoassembly) and processed into electrically conductive paper sheets60. As shown in FIG. 2A-D, the CNT-impregnated fabricated paper sheets60 are employed as the current collectors 15 in the cell. Lithiumtitanium oxide (LTO) and lithium cobalt oxide (LCO) are typically usedas respective-electrode materials 10. However, CNT 25 may be replacedwith any (typically nanoscale) inorganic or organic conductive materialsor combinations thereof, or even combinations of electrically conductiveand non-conductive materials. Typically, the electrode materials 15exhibit higher flexibility when supported by the fabricated paper basedcurrent collectors 15. Likewise, LTO and LCO can be replaced with otherelectrode materials, such as lithium magnesium oxide, lithium manganeseoxide, combinations thereof and the like.

Optionally, the microfibers 65 are derived from beaten, bleachedsoftwood microfibers (less than 1% lignin and 99% cellulose). Typically,these typically hollow microfibers are 0.1-100 mm in length, and, moretypically, about 0.1-5 mm in length. Typically, these microfibers 65 arebetween about 1 μm and about 200 μm in diameter, and more typicallybetween about 35 μm and about 200 μlm in diameter.

Typically, cellulosic (paper) fibers 65 are soaked in polyethylene oxide(PEO) solution (1 g/liter) for 5 hours. The soaked fibers are formedinto a paper through a filter and press method. It should be noted thatwhile paper is discussed in detail in this embodiment, any flexiblefibrous material may be used as a substrate 15.

Optionally, flexible solid electrolytes 70 may replace the separator 45and be generated from various concentrations and mixtures of an ionicconductive polymer such as: (PEO), polyethylene glycol (PEG),polyvinylidene fluoride, polyvinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP), polymethyl methacrylate, polyacrylonitrile, Polyacrilic acid(compolymers with PEG), PEG methacrylate polymers (PEGMA), or polymermodified emulsions (PME) and the like, and a lithium salt with voltagerange typically between about 0.5 volts to 4.5 volts such as lithiumhexafluorophosphate (LiPF6),), Lithiumbis(Trifluoromethanesulfonyl)imide (LiiTFSii), Lithium perchlorate(LiCL04), Lithium bis(perfluoroethylsulfonyl)imide, Lithiumbis(oxatlato)borate, Lithium triflate (LiCF3S04) and lithium-ionconductive glass-ceramics (LiCGC) powder including alumina, silica,titanium oxide (Ti02), and the like as precursors. Solvents, includingbut not limited to, acetonitrile and n-methyl-2-pyrrolidone (NMP) may beused in order to achieve a solution mixture of various combinations ofPEO, LiPF6, and LiCGC powder.

Typically, solid electrolytes 70 may be fabricated using PEO and LiPF6.Typically, 1 g of PEO and 0.4 g of LiPF6 are mixed with 30 mlAcetonitrile solution. Thin solid electrolyte films 10 of the mixtureare fabricated through various film fabrication methods such as spincasting, drop casting, spray coating, blade coating, or the like. Thefilms 10 are dried in air and/or vacuum. The thicknesses of suchfabricated solid electrolyte films 72 range from about ten micrometersto about five millimeters. However, thinner (to nanometer) or thickerelectrolyte films 10 may be fabricated and employed using the disclosedfabrication methods. Alternately, the solid electrolytes 70 may bepowdered and dispersed through the electrode portion 10, the currentcollectors 15, or both.

Optionally, solid electrolytes 70 are fabricated using PEO, LiPF6, andLiCGC powder. A few drops of N-Methyl-2-pyrrolidone (NMP) may be addedto 0.4 g of LiPF6 and stirred until completely mixed. The admixture maybe added to 40 ml of acetonitrile solution. While stirring, 0.1 g ofLiCGC powder may be added. Stirring is continued until a uniform mixtureis achieved. Further, 1 g of PEO may be added and stirred until auniform mixture is formed. Thin films of the mixture are fabricatedthrough various film fabrication methods such as spin casting, dropcasting, spray coating, blade coating, or the like. The films may bedried in air and/or vacuum. The thicknesses of such fabricated solidelectrolyte films range from ten micrometers to five millimeters.However, thinner (to nanometer) or thicker electrolyte films 10 can befabricated and employed using the disclosed fabrication methods.

The lithium metal oxide electrode portions 10 may be fabricated by anyone of a variety of methods. In one embodiment, pastes of the electrodematerials 10 are prepared using 85% (wt) active materials (lithiumtitanium oxide (Li4Tis0n LTO) or lithium cobalt oxide (LiCo02 or LCO)),10% (wt) conductive carbon black compounds such as carbon black,conductive carbon black and the like, and 5% (wt) polyvinylidenefluoride (PVDF). It may be possible to increase the conductivity of theelectrode paste while PVDF diluted in N-methyl-2 pyrrolidone (NMP) mayimprove adherence of the materials to the substrate. Each of theelectrode materials (LTO, LCO, and the like) may then be separatelycoated on at least partially electrically conductive current collectorsusing such methods as, spray coating, spin casting, drop casting andblade coating, typically followed by vacuum drying. Generally, the metaloxide coated current collectors are dried in vacuum for about 12 hours.

In one embodiment, a battery 5 is prepared with half-cells includeslithium foil as the anode 40 and aluminum or conductive paper coveredwith the LiMm04 as the cathode 35. The conductive paper 60 is preparedby applying a few layers of single wall carbon nanotube 25 with over thecellulose wood fibers 30, such as by a layer-by-layer process. Afterfabrication of the conductive paper 60 a layer of active material 10 isdeposited such as by a spray-coating technique. LiMm04 (85%) is mixedwith a conductive carbon black compound and KS6 (10%) andpPolyvinylidene fluoride (5%) to form a paste of active material.1-methyl-2-pyrrolidone was used as the solvent for the active materialmixture The mixture is coated both on the conductive paper 60 andaluminum substrates. The coated electrodes 12 are dried in an oven for5-6 hours at 60° C. and then transferred to an argon field glovebox. Theelectrodes 12 are cut in circles with diameter of ⅜ inch to fit in acoin cell. LiPF6 in EC/DEC solvent (1:1 vol) is used as an electrolyte.A polymer dielectric member is used as a separator 45, and positioned inbetween of the lithium foil and the cathode material to form half-cells.The cell may be stabilized for about 12 hours.

One typical battery 5 configuration of the present novel technology isshown in FIG. 1. The electrodes 12 (LTO or LCO deposited onnanomaterial-coated microfiber paper) are cut in circular samples.Typically, a diameter of between about 0.1 inches to about 0.5 inch isused, and more typically, a diameter of about 0.23 inch diameter isused. The electrodes 12 define the anode 40 and cathode 35 of thebattery 5. A polymer separator film 45 divides the anode 40 and cathode35 of the battery 5. Typically, a dielectric separator 45 is used toseparate the cathode portion 35 and the anode portion 40, moretypically, a polymer separator is used. The outer surfaces of thecurrent collectors 15 are connected in electric communication to theoutput of the battery 5. The device assembly is typically soaked inelectrolyte solution and packaged. In this example, the battery assembly5 is soaked in lithium phosphorous fluoride (LiPF6) 1M in an ethylenecarbonate/diethyl carbonate (EC/DEC) electrolyte solution (1:1 byvolume), pressed, and encapsulated in a coin cell. Alternately, thebattery 5 may be separately packaged into a pouch cell. Any convenientform of flexible or rigid packaging methods for the battery 5 may beutilized. The capacities of the batteries 5 made with the currentcollectors 15 may be measured to be 150 mAh/g for lithium cobalt oxide(LCO) half-celt 158 mAh/g for lithium titanium oxide (LTO) half-cell and126 mAh/g for the full-cell.

Flexible electrochemical device 5 typically does not appreciablydeteriorate or lose efficiency due to bending. In some embodiments, afibrous material 30 is infused with an at least partially electricallyconductive nanomaterial 20 to define a current collector 15. Theresultant combination may be coated or deposited with a lithium metaloxide electrode portion 10, or the like to define an electrode 12. Atleast two of these fabricated electrodes with different lithium metaloxide coatings may be separated by a dielectric separator 45 to define abattery. An electrolyte 70 may be dispersed or infused in the lithiummetal oxide electrode portion 10 and/or the fibrous nanomaterial currentcollector 15. The electrolyte 70 may be a solid or liquid. Some of thedemonstrated benefits of the novel system are detailed in the followingexamples.

Example 1

A battery system was prepared as detailed above. The electrodes werestudied under SEM after repeated bending. In one study, LCO and LTO werecoated on two separate CNT-microfiber paper samples. After vacuum dryingthe samples for 12 hours, they were bent repeatedly to a 300 degreeangle 20 times. The SEM images of the resulting samples are shown inFIGS. 2A (LCO) and 2B (LTO). For comparison, a similar test wasconducted on samples prepared from aluminum and copper foils. These SEMimages are shown in FIGS. 2C (LCO on aluminum (AI)) and 2D (LTO oncopper (Cu)). The insets of the figures highlight areas with observedsurface fractures. These images reveal that the electrode materiallayers on AI and Cu foils suffer from deep fractures over large areas.In comparison, defects on the electrode material coated on microfibercurrent collectors are relatively shallow and isolated to a small area.Furthermore, some parts of the active materials over the AI and Cu foilsare detached from the surface, introducing gaps under the activematerial layer. In contrast, no such effects were observed withCNT-microfiber paper current collectors. Although the surface has somefractures, the porous structure of the paper acts to keep the electrodematerial on the substrate, thus preventing the detachment of activematerial from the current collector.

LTO and LCO half-cells with CNT-microfiber paper current collectors weretested between 0.5 and 1.8 V and 3.5 to 4.3 V, respectively. Thecharging capacity of the LCO half-cell was measured to be 150 mAh/g andthe discharging capacity of the LTO half-cell was measured to be 158mAh/g at a Cj5 current rate (the term C specifies that the chargingcycle for the cell takes 5 hours with the given current). This capacityis comparable with the equivalent device fabricated from AI and Cucurrent collectors and tested under the same conditions. The resultsshow the developed cells are stable with a less than 1% drop in capacityfrom the first to the 15th cycle.

Full-cells with LTO and LCO electrodes on CNT-microfiber paper currentcollectors were tested for charging/discharging performance between 1.2V and 2.7 V. The maximum charging capacity was measured 126 mAh/gat aC/5 current rate. Further tests were conducted with higher C rates andthe charging capacities were measured to be 112 mAh/g for C/2.5 and 107mAh/g for C. The internal resistances of full-cells were measured to be2.8 O for paper-based and 1.95 O for metallic current collectors. It wasalso observed that, after 25 cycles, the charging capacity of thebatteries dropped by approximately 15%, mostly caused by the drop at thefirst charging cycle and attributed to the stabilization of the cell. Asimilar drop was observed on the first cycle in the devices withmetallic current collectors. The cell performance was observed to bestable after the first cycle.

The columbic efficiency of the battery was measured 84% for the firstcycle, which increased to 96% in the second cycle and stayed between96-98% thereafter. This is attributed to the fact that in the firstcycle the charging takes place between 0 and 2.7 V while in thefollowing cycles the discharge voltage is limited to 1 V, resulting in ahigher charging time and lower discharging time in the first cycle.After the first cycle, the battery reaches a stable state.Self-discharge results of the full-cell charged to 2.7 V at C/5 currentrate was tested. After 90 hours the battery output voltage was stable at2 V.

Example 2

Polyimide-polyethylene oxide/lithium hexafluorophosphate (PEO/LiPF6)solid electrolyte, and flexible solid electrolyte batteries were tested.Various methods of solid electrolyte fabrication using variousconcentration mixtures of PEO and LiPF6 were explored. One embodiment ofa battery configuration with the solid electrolyte is shown in FIG. 19.

In one exemplary fabrication of PEO/LiPF6 solid electrolyte, 1 g of PEOand 0.4 g of LiPF6 were mixed to yield an admixture, which was thenfurther ground with the help of mortar and pestle. The admixture wasmixed in 30 ml acetonitrile solution. The solution was then stirred for12 hours. At first the mixture was stirred at higher speed which wasthen slowed to prevent formation of air bubbles. Thin solid electrolytefilms of the mixture were fabricated through various film fabricationmethods such as spin casting, drop casting, and blade coating. The filmswere dried in air and in vacuum. The thicknesses of such fabricatedsolid electrolyte films ranged from 10 micrometers to 5 millimeters.However, thinner (to nanometer) or thicker electrolyte films can befabricated and employed using the disclosed fabrication methods.

The so-produced solid electrolytes may be used to fabricate batteriesusing appropriate electrode materials and battery assembly. In oneexample, batteries with LTO or LCO (or both) electrodes and PEO/LiPF6electrolyte were fabricated and tested. FIG. 3 shows the charging curveof one such battery.

In another example a few drops of liquid electrolyte were added to thePEO/LiPF6 solid electrolyte films to improve the performance of thebattery. The porous structure of PEO film assists in absorbing andtrapping the liquid electrolyte, increasing the ionic conductivity ofthe gel electrolyte. The charging curve of such a battery is shown inFIG. 4.

Example 3

Polyimide-polyethylene oxide/lithium hexafluorophosphatejlithium-ionconductive glass-ceramics (PEO/LiPF6/LiCGC) slid electrolyte andflexible solid electrolyte batteries were produced and examined. Variousmethods of solid electrolyte fabrication using various concentrationmixtures mixtures of PEO, LiPF6 and LiCGC powder, and flexible solidelectrolyte batteries were tested.

In one exemplary fabrication of PEO/LiPF6/LiCGC a few drops ofN-Methyl-2-pyrrolidone (NMP) were added to 0.4 g of LiPF6 and themixture was stirred until completely mixed to define an admixture. Theadmixture was added to 40 ml of acetonitrile solution. While stirring0.1 g of LiCGC powder was added. Stirring was continued to yield auniform mixture. One gram of PEO was then added and stirred until auniform mixture was formed. Thin solid electrolyte films of the mixturewere fabricated through various film fabrication methods such as spincasting, drop casting, and blade coating. The films were dried in airand in vacuum. In a further exemplary method the films were fabricatedhydrophobic substrate including heat-treated glass and silicon tape foreasy peeling off. In another exemplary method the films were dried in anacetonitrile environment to prevent contact with air. The thicknesses ofsuch fabricated solid electrolyte films ranged from 10 micrometers to 5millimeters. However, thinner (to nanometer) or thicker electrolytefilms can be fabricated and employed using the disclosed fabricationmethods.

In one example, the PEO/LiPF6/LiCGC electrolyte film was peeled off fromthe substrate before it was completely dry (semi solid) and it was usedin fabrication of solid lithium ion battery devices. Charging curve ofsuch a device is shown in FIG. 5.

In another method, the PEO/LiPF6/LiCGC electrolyte film was completelydried. The electrolyte film was then used in fabrication of solidlithium ion battery devices. The charging curve of such a device isshown in FIGS. 6A and 6B.

In another exemplary method the electrolyte material was applied on aheat-treated glass substrate. In an exemplary method the substrate washeated at 540° C. for an hour. The heat treatment of the glass substratemakes the surface hydrophobic which improves the peeling process of thedried electrolyte film. This method reduced the fragments in the polymerfilm and provided better polymer surface.

In another exemplary method the solid electrolyte was dried in anacetonitrile environment. In an exemplary method the solid electrolytefilm was dried in a glass desiccator filled with acetonitrile vapor. Abetter uniformity of the film was achieved.

In another example, the electrolyte film was dried in a vacuum oven. Inan exemplary method the electrolyte film was dried at 60° C. for 5hours. Drying of the film reduces the defects incurred during peeling ofthe process.

In another example the solid electrolyte films were directly applied onthe electrode material layers. In an exemplary method the electrolytemixture (PEO/LiPF6/LiCGC) was applied directly on the electrodematerials and were put together to form a battery device assembly. Abetter connection of electrolyte and electrode materials was observed.

Batteries with the solid electrolyte and paper-based flexible electrodeson nanomaterials-coated wood microfiber current collectors werefabricated and tested. A preliminary exemplary charging curve result isshown in FIG. 7.

One example of coating electrode material on paper-based currentcollector using blade coating method is disclosed. This process providesthicker electrode layer that help reduce pinch-through shorting problemseen in some battery devices.

In one example, a paste of electrode material with a few drops ofsolvent was prepared. The amount of active material was also increasedby 5% (wt) (to achieve a better surface structure). After mixing theelectrode martial with PVDF binder, KS6, and carbon conductive materialsin a mortar, a few drops of NMP was added to make a creamy paste. A 1-5mm thick layer of the paste was deposited on the current collectorthrough blade coating method. In a further exemplary method the sampleswere dried for 2-3 hours and were heated in a vacuum oven. In anexemplary method aluminum foil current collector was used. The images ofthe dried and cut samples are shown in FIG. 8. In an exemplary methodCNT-coated wood microfiber paper current collector was used, shown inFIG. 9.

In another exemplary method hot press was used to for the cell assembly.In this method a hot press with a temperature between 90-150° C.(slightly lower than melting point of the polymer separator) was used toprovide a multilayer structure of theanode/separator-electrolyte/cathode. The use of hot press provides abetter integration of different layers of battery and a betterattachment between the surface of the polymer electrolyte and activematerials. This also leads to lower internal resistance inall-solid-state batteries.

Example 4

Poly(vinylidene fluoride)/poly Ethylene Glycol/polyimide-polyethyleneoxide/lithium bis(Trifluoromethanesulfonyl)imide (PVDF,PEG,PEO,LiTFSI)solid electrolyte was also fabricated and tested. The gel electrolyteswere produced using 1:1:2 wt. % in 5 ml of acetonitrile solution. Thesalt was added to the solution and stirred for 12 hours. The mixture wascasted using blade-coating method to form a thin membrane. The membranewas dried using ultraviolet light for two hours. It was then transferredto a vacuum oven for a drying process where it was kept for 12 hours at50° C. The ionic conductivity of the film was measured to be 10-⁵ S-cm,as shown in FIG. 20.

In another exemplary method a solid electrolyte was prepared using anionic conductive polymer such as polyvinylidene fluoride orpoly(vinylidene fluoride-co-hexafluoropropylene) and a lithium salt suchas lithium bis(Trifluoromethanesulfonyl)imide (LiTFSi), lithiumtrifluoromethanesulfonate (LiTF), or lithium perchlorate (LiCL04). Thepurpose of the salt is to improve the ionic conductivity of themembrane. To further enhance the ionic conductivity of the member afiller material such as lithium-ion conductive glass-ceramics (LiCGC)salt or other conductive polymer such as polyethylene glycol (PEG) arealso added to the mixture. The filler material improves the ionicconductivity of the film by generating empty spaces in the polymer filmwhich serve as ion traps for the lithium slat. Different concentrationsof the fillers and polymers can provide different structures, and thusdifferent ionic conductivities.

Example 5

Paper based lithium ion batteries using current collectors made ofcarbon nanotube infused paper were constructed and tested. The pulp usedin the experiments was made from beaten, bleached softwood microfibers(less than 1% lignin and 99% cellulose), press-dried, and shipped inbundles of 17″×14″ sheets. These hollow microfibers are 0.1-5 mm inlength and 5-200 μm in diameter.

An aqueous dispersion ofpoly(3A-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS)conductive polymer (3 mg/ml) and carbon nanotubes (25 pg/ml) was used asthe anionic component, while poly(ethyleneimine) (PEI) (3 mg/ml) wasused as cationic polyelectrolyte component for the LbL coating of thewood microfibers. Coating microfibers with two bi-layers of (PEI/CNT) inalternate with one bi-layer of (PEI/PEDOT-PSS) achieved the desiredconductivity. Following CNT-coating, the wood microfibers were assembledinto flexible paper sheets (through an in-house setup made in accordanceto the Technical Association of Pulp and Paper (TAAPI) T 205T standard)to be used as current collectors.

Pastes of the electrode materials were prepared using 85% (wt) of theactive materials (Li4Ti5012 or LiCo02), 10% (wt) conductive materialmade from carbon black with a particle size of about 40 nm, and 5% (wt)Polyvinylidene fluoride (PVDF). Conductive carbon black increases theconductivity of the electrode paste while PVDF diluted in N-Methyl-2Pyrrolidone improves adherence of the material to the substrate[10]-[11]. The electrode materials were then coated on the currentcollectors by a spray coating method followed by vacuum-drying for 12hours.

The battery configuration was assembled in an argon filled glove box.The CNT-microfiber paper, coated with lithium titanium oxide (LTO) andlithium cobalt oxide (LCO), are cut in circular samples with diameter of0.23 inches, which form the anode and cathode of the developed battery.A separator film divides the anode and cathode of the battery. The outersurfaces of the CNT-microfiber current collectors are connected to theoutput of the battery. The battery assembly was soaked in lithiumphosphorous fluoride (LiPF6) 1M in an EC/DEC electrolyte solution (1:1by volume), pressed, and encapsulated in a Coin cell. After assembly,the devices were kept undisturbed for 24 hours allowing them to reachstable states by completely soaking the electrode materials and theseparator with the electrolyte solution. LCO/LTO half-cells andfull-cells were tested using a battery testing system.

The resistivity of the microfibers was measured after each bi-layercoating of PEI/PEDOT-PSS and PEI/CNT in the Lbl nanoassembly process.The measured resistivity results are shown in FIG. 11. The conductivityof fibers is measured using a micro-probing station at 0.5 to 5 V. Asexpected, results show that resistivity decreased following the additionof each bi-layer of PEI/PEDOT-PSS or PEI/CNT on the microfibers. It wasalso observed that after a slight increase for PSS/PEI precursor layers,the resistivity of the microfibers decreases exponentially with theaddition of PEI/PEDOT-PSS and PEI/CNT bi-layers. A layer of PEDOT-PSSsandwiched between CNT layers forms a thin layer, of about 2 to 5 nm,around the fibers and enhances the electrical characteristics byreducing the resistivity of the coated fiber. This is likely due to theelectrical path provided by PEDOT-PSS, which forms a continuousconductive network between CNT on the surface of the wood microfibers.It has previously been shown that the PEDOT-PSS makes conductive jointsbetween CNT-PSS smoother by increasing the clusters. In addition, as CNTis expected to dominate the conductive path, even though PEDOT is knownfor oxidation, its effect on the conductivity of the paper is minimal.

SEM images of the uncoated and coated microfibers reveals surfacestructures, as shown in FIG. 12A-B. FIG. 12A shows the surface of anuncoated microfiber, while FIG. 12B shows the surface of a microfibercoated with two hi-layers of (PEijCNT) in alternate with one bi-layer of(PEI/PEDOT-PSS). The thickness of the paper sheet was measured to beabout 50 μm. The resistivity of a CNT-microfiber paper sheet fabricatedfrom these coated wood microfibers was measured to be 1.92 kQ-cm. Theconductivity results show that further improvement is needed for highcurrent applications.

Cross-sectional SEM images of the LCO and LTO coating on CNT-microfiberpaper current collectors are presented in FIG. 13A-B. As shown, theelectrode material layers (LTO and LCO) that consist of activematerials, PVDF, and conductive carbon black, form a solid layer on thetop of current collectors. It can also be noted that the CNT-microfiberpaper current collector and electrode material layers are discreet withsome interpenetration. The latter helps the electrode material layers tobe intact on the surface of the developed CNT-microfiber paper currentcollectors.

The developed paper-based batteries have an inherent flexibility.Therefore, the performance under conditions of repeated bending wasstudied. In this study, LCO and LTO were coated on two separateCNT-microfiber paper samples. After vacuum-drying the samples for 12hours, they were bent to a 300 degree angle 20 times. The SEM images ofthe resulting samples are shown in FIGS. 2A (LCO) and 2B (LTO). Forcomparison, a similar test was conducted on samples prepared fromaluminum and copper foils. These SEM images are shown in FIGS. 2C (LCOon AI) and 2D (LTO on Cu). The insets of the figures highlight areaswith observed surface fractures. SEM images reveal that the electrodematerial layers on AI and Cu foils suffer from deep fractures over largeareas. In comparison, fractures on newly developed microfiber currentcollectors are shallow and contained. Furthermore, some parts of theactive materials over the AI and Cu foils are detached from the surface,introducing gaps under the active material layer. In contrast, no sucheffects were observed with CNT-microfiber paper current collectors.Although the surface has some fractures, the porous structure of thepaper, keeps the material on the substrate, preventing the detachment ofactive material from the current collector.

Standard LTO and LCO half-cells using Cu and AI current collectors weretested between 0.5 V to 1.8 V and 3.5 V to 4.3 V, respectively. It wasobserved that the charging capacity of the LCO half-cell is 149 mAh/g,and the discharging capacity of the LTO half-cell is 156 mAh/g at C/5.

The mass loading of the active materials on the anode (LTO) and cathode(LCO) are 8.8 mg/cm² and 9.2 mg/cm², respectively. The CNT mass loadingon the current collectors is 10.1 g/cm². The LTO and LCO half-cells withCNT-microfiber paper current collectors were tested between 0.5 and 1.8V and 3.5 to 4.3 V, respectively. The charging/discharging capacitycurves for LCO and LTO half-cells are shown in FIGS. 14A and 14B,respectively. The performances of the batteries for the first 15 testcycles are presented. It is observed that the charging capacity of theLCO half-cell is 150 mAh/g and the discharging capacity of the LTOhalf-cell is 158 mAh/gat a C/5 current rate. This capacity is comparablewith the equivalent device fabricated from AI and Cu current collectorsand tested under the same conditions. The results show the developedcells are stable with a less than 1% drop in capacity from the first tothe 15th cycle.

The full-cells with LTO and LCO electrodes on CNT-microfiber papercurrent collectors were tested for charging/discharging performancebetween 1.2 V and 2.7 V. The galvanostatic charging/discharging curvesand capacity measurements for the first cycle are shown in FIG. 15A. Themaximum charging capacity was measured to be 126 mAh/g at a C/5 currentrate. Further tests were conducted with higher C rates and the chargingcapacities were measured to 112 mAh/g for C/2.5 and 107 mAh/g for C. Theinternal resistances of full-cells measured through a Nyquist plot wereobserved to be 2.8 O for paper-based and 1.95 O for metallic currentcollectors. It was also observed that, after 25 cycles, the chargingcapacity of the batteries dropped by approximately 15%, mostly caused bythe drop at the first charging cycle and attributed to the stabilizationof the cell. A similar drop was observed on the first cycle in thedevices with metallic current collectors. The cell performance wasobserved to be stable after the first cycle.

The columbic efficiency of the battery, as shown in FIG. 15B, wasmeasured to be 84% for the first cycle, which increases to 96% in thesecond cycle and stays between 96-98% thereafter. This is attributed tothe fact that in the first cycle the charging takes place between 0 and2.7 V while in the following cycles the discharge voltage is limited to1 V, resulting in a higher charging time and lower discharging time inthe first cycle. After the first cycle, the battery reaches a stablestate. Self-discharge results of the full-cell charged to 2.7 Vat C/5current rate is shown in FIG. 16. It can be noted that after 90 hoursthe battery output voltage was stable at 2 V.

Example 6

A paper based lithium manganese oxide battery was constructed andtested. The half-cells were made using lithium foil as anode andaluminum or conductive paper covered with the LiMm04 as the cathode. Theconductive paper is prepared by applying a few layers of single wallcarbon nanotube with over the cellulose wood fibers throughlayer-by-layer process. After fabrication of the paper a layer of activematerial was deposited through spray-coating method. LiMm04 (85%) wasmixed with conductive carbon black and KS6 (10%) and polyvinylidenefluoride (5%) to form a paste of active material. 1-Methyl-2-pyrrolidonewas used as the solvent for the active material mixture. This mixturewas coated both on the conductive paper and aluminum substrates. Thecoated electrode was dried oven for 5-6 hours at 60° C. and thentransferred to an argon field glovebox. The electrodes were cut incircles with diameter of ⅜ inch to fit in a coin cell. LiPF6 in EC/DECsolvent (1:1 vol) was used as electrolyte. A separator was used as aseparator, placed in between of the lithium foil and the cathodematerial to form half-cells. The cell was allowed to stabilize for 12hours and was subsequently tested using with a battery testing device.

Both aluminum and paper based half-cells were charged between 2.5 V to4.5 V at 0.2 rnA current rate. The galvanostatic charging/dischargingcapacity results for the paper based and aluminum current collectorhalf-cells, measured separately, are shown in FIGS. 17A and 17B,respectively. The results show that the performance of the battery ofpaper-based current collector is comparable with the aluminum baseddevice. The results from both the measurements are compared and thecharge capacity of paper based half-cell was measured to be 130 mAh/gand for aluminum based half-cell it was measured to be 129.5 mAh/g.

To check the stability of the active materials on the paper currentcollector, SEM images of the electrodes were taken. FIG. 18 A shows theSEM image of active material on the paper current collector. The crosssection showing the active material and the coated cellulose wood fiberof the conductive paper current collector is presented in FIG. 18B.

While the novel technology has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character. It is understood thatthe embodiments have been shown and described in the foregoingspecification in satisfaction of the best mode and enablementrequirements. It is understood that one of ordinary skill in the artcould readily make a nigh-infinite number of insubstantial changes andmodifications to the above-described embodiments and that it would beimpractical to attempt to describe all such embodiment variations in thepresent specification. Accordingly, it is understood that all changesand modifications that come within the spirit of the novel technologyare desired to be protected.

What is claimed is: 1) A flexible, paper based electrochemical devicecomprising: first and second fibrous sheets, each respective sheet atleast partially infused with electrically conductive nanomaterial; adielectric sheet; a first lithium-metal oxide electrode sheetoperationally disposed between the first fibrous sheet and thedielectric sheet; and a second lithium-metal oxide electrode sheetoperationally disposed between the second fibrous sheet and thedielectric sheet; wherein the first lithium-metal oxide electrode sheetand the second lithium-metal oxide electrode sheet have differentcompositions. 2) The device of claim 1 wherein the first and secondlithium-metal oxide electrode sheets are selected from the groupincluding: lithium titanium oxide, lithium cobalt oxide, lithiummagnesium oxide, lithium manganese oxide, any other lithium salt with adesired voltage of about 4.5 V, and combinations thereof. 3) The deviceof claim wherein at least one lithium-metal oxide electrode sheet isinfused with an electrolyte. 4) The device of claim 1 wherein at leastone at least partially electrically conductive nanomaterial infusedfibrous sheet is infused with an electrolyte. 5) The device of claim 1further comprising at least one lithium-metal oxide electrode sheet andat least one at least partially electrically conductive nanomaterialinfused fibrous sheet are infused with an electrolyte. 6) The device ofclaim 5 wherein the electrolyte is a solid. 7) The device of claim 1,and further comprising a housing positioned around the first and secondfibrous sheets. 8) The device of claim 1 wherein the paper-basedelectrochemical device is flexible. 9) The device of claim 1 wherein thepaper-based electrochemical device is rechargeable. 10) The device ofclaim 1, wherein the at least partially electrically conductivenanomaterial infused fibrous sheet is cellulosic paper. 11) The deviceof claim 1 wherein the at least partially electrically conductivenanomaterial is a plurality carbon nanotubes. 12) An electrochemicalcell comprising: a first electrically conductive carbon nanotube infusedpaper member; an electrically nonconductive separator member; a firstlithium metal-oxide member sandwiched between the first electricallyconductive carbon nanotube infused paper member and the separatormember; and an electrolyte dispersed in the lithium metal oxide member.13) The electrochemical cell of claim 12, further comprising a secondlithium-metal oxide member sandwiched between a second electricallyconductive carbon nanotube infused paper member and the dielectricmember to define an electrochemical device; wherein the separator memberis an electrically nonconducting dielectric material. 14) Theelectrochemical cell of claim 12 wherein the separator is an electrolytegel. 15) A method for fabricating a paper lithium ion cell comprising:a) depositing a first lithium-metal oxide composition onto a firstelectrically conducting microfiber paper substrate to define a cathode;b) depositing a second, different lithium-metal oxide composition onto asecond electrically conducting coated microfiber paper substrate todefine an anode; c) separating the cathode and the anode with a barriermaterial; d) infusing the cathode and the anode with electrolytes; ande) encapsulating the anode, the cathode, and the barrier material in ahousing. 17) The method of claim 16 and further comprising: f) beforea), impregnating the first and second electrically conducting microfiberpapers with carbon nanotubes. 18) The method of claim 16, wherein thefirst and second electrically conducting microfiber papers are formedfrom cellulosic microfibers between about 0.1 mm and about 5 mm inlength. 19) The method of claim 18 wherein the microfibers havediameters between about 35 micrometers and about 200 micrometers. 20)The method of claim 16 wherein the first and second lithium-metal oxidecompositions are selected from the group including: lithium titaniumoxide, lithium cobalt oxide, lithium magnesium oxide, lithium manganeseoxide, any other lithium salt with a desired voltage of about 4.5 V, andcombinations thereof.